1. Field of the Invention
The present invention relates to a vibration type (vibratory wave) driving apparatus for generating vibration (vibratory wave) in a vibratory body by vibration of piezoelectric element and utilizing the vibration energy to impart driving force to a moving body. More particularly, the invention concerns control of this vibration type driving apparatus.
2. Related Background Art
There are many conventional proposals concerning the control method of such vibration type driving apparatus, and a typical example is a method for controlling operation speed by changing the frequency or the amplitude of a voltage signal (drive signal) applied to the piezoelectric element.
Here, the relationship between the frequency and amplitude of drive signal and the operation (rotation) speed is as shown in FIG. 12. Namely, there are such characteristics that the rotation speed drops gently on the higher frequency side but drops steeply on the lower frequency side with respect to the peak of the resonance frequency of the vibrator. In addition, there are also such characteristics that the rotation speed increases as the amplitude of the drive signal increases.
Incidentally, often used for controlling the speed by change of frequency (hereinafter called frequency speed control) is a VCO (Voltage-Controlled Oscillator) capable of attaining relatively fine frequency resolution against input voltage. However, from the aspect of cost, use of digital circuitry (gate array) is more preferred than use of analog circuitry such as the VCO. In the case of the gate array being used, the frequency of the clock signal determines the frequency of the drive signal and there is a limit for the clock frequency. Therefore, it is not possible to achieve such high frequency resolution as that of VCO. As a result, the speed of the vibration type driving apparatus is controlled only stepwise, which would result in a drawback of easily causing large speed vibrations.
On the other hand, when the speed is controlled by changing the amplitude (hereinafter called amplitude speed control), the control can be performed with less speed vibrations as compared with the frequency speed control with digital circuitry. In order to carry out accurate speed control by the amplitude speed control, it is, however, necessary to keep the drive frequency constantly set at a frequency higher than and close to the resonance frequency. Since the resonance frequency varies with change in the environment including the temperature and change of load, this method has a drawback that accurate speed control is not easy.
Under such circumstances, there are proposals of the following techniques. For example, (1) Japanese Laid-open Patent Applications No. 3-239168 and No. 4-222476 describe such a technique that the vibration frequency of the vibration type driving apparatus is detected and control is made so as to keep the frequency of drive signal always near the resonance frequency and that as for the operation speed, the amplitude of drive signal is changed so as to decrease a difference from a target speed as detecting actual operation speeds. Further, (2) Japanese Laid-open Patent Application No. 6-237584 describes such a technique of control of the operation speed that upon start only the frequency is changed to take the operation speed to near the target speed and thereafter only the amplitude of drive signal is controlled. These techniques as described in the above bulletins enable control with less speed variations by digital circuitry.
However, the above technique of (1) has a problem of cost increase because of a need for means for detecting whether the vibration frequency is kept near the resonance frequency, in addition to means for detecting the operation speed.
Further, the above technique of (2) has problems that the control becomes complex and that the operation of the driving apparatus becomes unstable upon changeover because of a need for changeover between the frequency speed control and the amplitude speed control. Further, the above technique of (2) also has a problem of incapability of reacting against the change of the resonant point of the vibrator as described previously, because the drive frequency is fixed after the real speed becomes close to the target speed.